A switch-mode power converter (also referred to as a “power converter” or “regulator”) is a power supply or power processing circuit that converts an input voltage waveform into a specified output voltage waveform. DC-DC power converters convert a dc input voltage into a dc output voltage. Controllers associated with the power converters manage an operation thereof by controlling the conduction periods of switches employed therein. Generally, the controllers are coupled between an input and output of the power converter in a feedback loop configuration (also referred to as a “control loop” or “closed control loop”).
Typically, the controller measures an output characteristic (e.g., an output voltage, an output current, or a combination of an output voltage and an output current) of the power converter, and based thereon modifies a duty cycle of the switches of the power converter. The duty cycle is a ratio represented by a conduction period of a switch to a switching period thereof. Thus, if a switch conducts for half of the switching period, the duty cycle for the switch would be 0.5 (or 50%). Additionally, as voltage or current for systems, such as a microprocessor powered by the power converter, dynamically change (e.g., as a computational load on a load microprocessor changes), the controller should be configured to dynamically increase or decrease the duty cycle of the switches therein to maintain an output characteristic such as an output voltage at a desired value.
In an exemplary application, the power converters have the capability to convert an unregulated input voltage, such as 12 volts, supplied by an input voltage source to a lower, regulated, output voltage, such as 2.5 volts, to power a load. To provide the voltage conversion and regulation functions, the power converters include active power switches such as metal-oxide semiconductor field-effect transistors (“MOSFETs”) that are coupled to the voltage source and periodically switch a reactive circuit element such as an inductor to the voltage source at a switching frequency that may be on the order of 500 kHz or higher.
A conventional way to construct a power converter is to integrate several key semiconductor devices such as control circuit elements and an active semiconductor switch in an integrated circuit, and to couple the integrated circuit to separate components that are not easily integrated. Separate components that are not easily integrated, such as a power diode, typically dissipate a substantial level of power, or are formed of materials and structures that are not easily integrated in a semiconductor device, such as an output filter inductor or an output filter capacitor. Integrating a low-side power switch increases the size of the integrated circuit and increases the risk of latch up. Using a diode separate from the integrated circuit is a safer alternative in some designs.
In certain applications wherein a power diode is externally coupled to an integrated circuit, it is important to detect a diode open-circuit condition, caused either by a failed connection or a failed diode, and if the diode circuit is open, to power down the power converter. Failure to power down the power converter in such a circumstance can result in dangerously high pulsed voltages across terminals coupled to the diode when current flowing through the output filter inductor reverses direction, for example, during a low-load condition. Thus, in an application of a power converter such as in an automotive environment wherein a diode is separately coupled to an integrated circuit, if the diode is disconnected for any reason, this condition must be detected and the power converter shut down.
Switch-mode power converters are often constructed with active switches such as field-effect transistors to perform an output rectification function. In such arrangements, the switches are alternately controlled so that at least one switch is enabled to conduct at any time during a switching cycle (with suitable accommodation to prevent switch conduction overlap during the switching transitions). Such an arrangement is usually referred to as a “synchronous output stage,” and the process is referred to as “synchronous rectification” due to synchronization of conduction periods of the active output rectifying switches with conduction periods of switches on the primary side of a power converter when an isolation transformer is employed to metallically isolate the output terminals of the power converter from the input terminals.
A synchronous output stage generally allows simplification of the design of the controller if the power conversion mode of operation is limited to continuous conduction mode (“CCM”). CCM allows reversed current flow in the output inductor with a minimum load current of zero, even at maximum input voltage.
Thus, there is a need for a process and related method to detect an open-circuit condition for a rectifying diode separated from an integrated circuit in a switch-mode power converter. The detection process would shut down the power converter when such an open-circuit condition is detected, thereby providing safe operation for the power converter while admitting the economy and other advantages of separating an output rectifying diode from an integrated circuit.